1. Field of the Invention
The present invention relates to a filter coefficient generator, and more particularly, to a filter coefficient generator that promptly generates necessary filter coefficients in response to frequent sampling rate (frequency) changes of digital signals.
2. Background of the Related Art
Supporting various display modes (e.g., PIP, POP, Zoom-In, Zoom-Out, and etc.) and display formats have become essential factors for digital televisions of these days. Up/down sampling rate conversion techniques are often used when it is necessary to change image sizes in order to make proper changes to the various display modes/formats in the digital televisions.
FIGS. 1 and 2 show general up/down sampling rate conversion techniques widely used to make such changes.
FIG. 1A illustrates a 1:M up sampling process of a digital signal when M is equal to 3. After M-1 zero-stuffing step is performed, wherein M-1 zero-valued samples are added, the signal passes through a low-pass filter having its cutoff frequency equal to 0.5/M.
FIG. 1B illustrates a N:1 down sampling process of a digital signal when N is equal to 3. It consists of a low-pass filter having its cutoff frequency of 0.5/N and a decimator. The purpose of the low-pass filter is to prevent aliasing.
However, many practical applications require not only a simple 1:M or N:1 sampling rate conversion, but rather a general M:N rate conversion. Since the sampling locations of input and output data are different, and the up or down sampling step needs to be performed depending on the values of M and N, a M:N decimator is combined with a liner interpolator forming a general format converter (GFC). FIG. 2 illustrates the structure of a GFC, and an example of a 3:2 down sampling process. As shown in FIG.2, the sampling locations of input and output data are not identical. The output data are calculated by interpolating two adjacent input samples (or data).
A digital filter is an essential component in the process of changing display modes or image formats having various sizes, and the coefficients of the filter must be changed depending on the sizes of input and output data. Therefore, digital televisions, wherein the output sizes and display modes are frequently changed, require an automatic filter coefficient generator. Filter coefficient generators according to the prior art, however, are not able to timely generate filter coefficients necessary for the changes such as display modes and image formats. Therefore, the image quality may be impaired in a transition period.